It has heretofore been known that a catalyst system, composed of a solid catalyst component containing a magnesium atom, a halogen atom, and a titanium atom, together with an organoaluminum compound, is an olefin polymerization catalyst of high activity.
It has been desired to produce an ethylenic polymer having a narrow distribution of molecular weight, which is suitable for use in injection molding and which can provide film having excellent optical characteristics. However, for the ethylenic polymer prepared by polymerizing ethylene alone or copolymerizing with .alpha.-olefin with a catalyst system which is believed to provide a narrow distribution of molecular weight, solvent extraction analysis using a solvent such as cyclohexane indicates that there is a significant amount of extract (extremely low molecular weight polymers). The presence of such extremely low molecular weight polymers causes problems such as fuming, nozzle dirt (dirt formed by the oozing of extremely low molecular weight polymers from molds) and unpleasant odor, during the process of molding of the ethylenic polymer. Furthermore, in the course of the polymerization of the ethylenic polymer, fouling (i.e., attachment of polymers to the walls of a reactor and other apparatuses) or bridging in a hopper in the after-treating step (e.g., pelletizing step) occurs.
These phenomena are more prominent in the case of copolymers prepared by copolymerizing ethylene with .alpha.-olefin. Particularly, in the case of medium density polyethylene comprising ethylene and a relatively large amount of .alpha.-olefin, and low density polyethylene, the extract content using a solvent such as n-hexane increases. This extract is composed of extremely low molecular weight polymers and extremely low density polymers. The amount of the extremely low density portion is determined by the breadth of the distribution of densities (the distribution of degrees of branching) which occurs at the copolymerization.
Furthermore, with the ethylenic polymer thus-formed, the distribution of the particle diameter is broad, and particularly, there is a large amount of fine powdery polymer. This fine powdery polymer causes the choking of a filter at a subsequent step such as drying, and furthermore causes problems in transferring the ethylenic polymer. In some cases, it may cause a powder explosion.
Japanese Patent Application (OPI) Nos. 73991/1977 (the term "OPI" as used herein refers to a "published unexamined Japanese patent application"), 76291/1977, 112984/1979 and 30983/1975 disclose processes for the polymerization of .alpha.-olefin, principally propylene, by the use of catalyst components which are prepared using an ether compound. These processes are directed to the production of stereoregular polymers having high crystallinity, and there are no disclosures concerning the homopolymerization of ethylene or the copolymerization of ethylene with .alpha.-olefin. When propylene is homopolymerized by the use of the solid catalyst components of the present invention, only traces of polymers are obtained. On the other hand, when ethylene is homopolymerized or ethylene with .alpha.-olefin is copolymerized by the use of the catalyst components as disclosed in the foregoing Applications, the distribution of molecular weight is broad and the distribution of density is broad and, therefore, a relatively large amount of extremely low molecular weight polymer is obtained. Thus, it is apparent that the catalyst systems as disclosed in the foregoing patents are basically different from that of the invention.
U.K. patent application No. 2,008,131 discloses the use of an ether compound as an electron donor compound in the preparation of a solid catalyst component. In this case, however, the amount of the ether compound used is less than that of the present invention. In this U.K. patent application, an organometal compound containing alkali metal or alkali earth metal and aluminum as a cocatalyst is used. In this U.K. patent application, the homopolymerization of an olefin is mainly described, and it is described that the polymerization catalyst has excellent performance. When ethylene is homopolymerized or ethylene with .alpha.-olefin is copolymerized by the use of a catalyst system wherein the organoaluminum compound of the present invention is used in place of the organoaluminum compound used in this U.K. patent application, polymers containing a large amount of extremely low molecular weight polymers are obtained as in the case as described hereinbefore.
Japanese Patent Application (OPI) No. 30983/1975 discloses a treatment of a solid component containing anhydrous magnesium chloride, an electron donor compound, titanium halogen compound, etc., with ethers. The ether compounds used in this method, however, are limited to chain-like ethers and, furthermore, the amount of the ether compound used is markedly less than that of the present invention.
U.S. Pat. No. 3,989,881 describes the preparation of a catalyst component using a cyclic ether and a production of propylenic polymers by the use of the catalyst component. In the preparation of the catalyst component, titanium tetrachloride and magnesium halide are coprecipitated with ethers, which is different from the solid catalyst component of the present invention.
U.S. Pat. No. 4,190,555 discloses a treatment using an ether compound as an oxygen-containing organic compound and a method of treating titanium halide. This method provides a polymerization catalyst component of .alpha.-olefin and particularly it discloses a method of treating titanium tetrahalide and a method of aging titanium tetrahalide. In this method, however, it is required that the catalyst component fomred is colored from black-brown to black-purple. On the other hand, the solid catalyst component of the present invention is from white to gray or pink. Thus, the catalyst system of this method is different from that of the present invention.
French Pat. No. 2,392,038 discloses a method of treating using an electron donor compound to improve the catalyst activity and the stereoregularity. The amounts of the titanium compound and electron donor compound which can be used in the preparation of the catalyst component are greatly different from that of the present invention. This indicates that the catalyst system of this French Patent is a highly active catalyst for the polymerization of .alpha.-olefin (e.g., propylene), but that when it is used for the homopolymerization of ethylene or the copolymerization of ethylene with .alpha.-olefin, polymers having a broad distribution of molecular weight and a broad distribution of density are formed as in the case as described hereinbefore.
The catalyst systems as described in the foregoing literatures are to be used in the polymerization of .alpha.-olefin (principally, propylene) and they not only have excellent polymerization activity, but also provide good stereoregularity. However, ethylene homopolymers or ethylene-.alpha.-olefin copolymers prepared by the use of such catalyst systems contain therein a relatively large amount of extremely low molecular weight polymers. This demonstrates that catalyst systems which are effective in the polymerization of .alpha.-olefin are not always effective in the homopolymerization of ethylene or in the copolymerization of ethylene with .alpha.-olefin.